We have created an allelic PTHrP-lacZ knockin mouse that provides a simple and high-sensitivity readout of PTHrP gene expression in a wide variety of locations. One novel finding in this mouse is abundant PTHrP/? galactosidase expression in the periosteum. Another is abundant PTHrP expression in muscle-periosteal insertion sites, fibrous tendon and ligament insertion sites, and fibrocartilagenous tendon insertion sites. The present proposal comprises three specific aims designed to initiate studies of PTHrP function in these sites. Aim 1. We will characterize PTHrP, PPR and cell marker expression in selected periosteal sites and entheses in order to a) identify the cells that participate in paracrine signaling in these sites as well as their neighbors, b) enhance our descriptive appreciation of potential PTHrP functions in these sites, and c) enable the functional experiments proposed in Aim 2 and 3. Aim 2. Here, we propose two subaims, one designed to study PTHrP effects and gene regulation in the periosteum and the second to study the putative regulation of PTHrP expression in entheses by mechanical force. The periosteal subaim comprises two experimental designs, including 1) manipulation of periosteal appositional growth and it putative effects on PTHrP expression via growth hormone or a growth hormone receptor antagonist and 2) evaluating the potential effects of periosteal tension on PTHrP expression and appositional growth. The enthesis subaim comprises three experiments designed to load or unload selected entheses in order to test the relationship between mechanical force and PTHrP expression in these sites. These include a) PTHrP-lacZ crosses to mouse lines with muscular dystrophy or myostatin deficiency that represent models with markedly decreased or increased muscle mass, respectively, and b) sciatic neurectomy and c) tail suspension as unloading techniques. Aim 3. We will perform a conditional knockout of PTHrP signaling in the periosteum and selected tendon and ligament targets using the mouse periostin gene promoter to drive Cre. [unreadable] [unreadable] [unreadable]